Measuring the time of flight of photons may be used to determine the distance to a target, as disclosed by U.S. Patent Application Publication No. 2013-0077082 to Mellot. The device emits periodic infrared laser flashes toward a target. The photons reflected from the target return to a single photon avalanche diode (SPAD) array. When a SPAD is reached by a photon, it is set in an avalanche mode and produces an electric pulse. The flight time is determined by measuring the delay between the emission of the laser flash and the production of corresponding pulses by the SPAD array. Knowing the speed of light, the distance of the target is deduced from the time of flight.
FIG. 1 is a block diagram of the time of flight measurement circuit described in the '082 patent application. The circuit includes a phase comparator 10 that receives the pulses generated by a SPAD array 12, and a half-wave signal H produced by a variable delay line 14. The delay line 14 produces the signal H by delaying a reference signal Href based on a set point produced by an integrator 16. The integrator 16 receives the output of the phase comparator 10. The circuit thus forms a delay locked loop or DLL.
In practice, the phase comparator 10 and the integrator 16 are formed by a charge pump that charges or discharges a capacitor with the pulses produced by the array 12, depending on whether the pulses occur before or after a transition of the signal H. The circuit is thus configured to place the transition of the signal H so as to equalize the numbers of pulses occurring before and after the transition.
FIG. 2 is a timing diagram illustrating an example of evolution of the signals used by the measuring circuit of FIG. 1, when the loop is locked. An intermittent laser flux is emitted at the rhythm of active phases of a periodic excitation signal LP. A signal SPAD illustrates an example of corresponding response pulse bursts produced by the array 12.
Ideally, the envelope of the response pulse bursts reproduces the excitation signal LP with a lag. In practice, the pulses have a certain probability to comply with the expected envelope, but many photons fail to reach the array, and some arrive outside the expected envelope. As shown, some photons may arrive early because they are reflected by parasitic elements closer than the target, or arrive late after multiple reflections. Such “off limits” pulses may also come from ambient light.
The reference signal Href is a square wave having the same period as the excitation signal LP, whose rising transitions are centered in the flash emission phases. The signal H corresponds, when the loop is locked, to the signal Href delayed such that its rising transitions are centered in the bursts. The delay of signal H relative to signal Href is the sought time of flight ToF, and corresponds to the current set point provided by the integrator 16 to the variable delay line 14. The circuit of FIG. 1 is analog and has many components, such as the variable delay line and the charge pump, which may drift with temperature changes.